Cleaning process

ABSTRACT

Insulating liquids contaminated with suspended particles and dissolved material are regenerated by initially removing the suspended particles by triboelectric electric filtration and thereafter removing the dissolved material by adsorption of the dissolved material onto a suitable surface.

United States Patent Sam 1 1*Sept. 30, 1975 1 1 CLEANING PROCESS [75] Inventor: Masamichi Sato, Asaka, Japan [561 References Cited {73] Assignee: Xerox Corporation, Stamford, UNITED STATES PATENTS Conn 2,549,698 4/1951 Mason 210/243 3,231,324 1/1966 Young.... 210/503 X 1 Notice: The portion of the term of this 3,544,458 12/]9'70 Sato 210/65 patent subsequent to Dec. 1, 1987, has bccn dlsclaimflt Primary E.\zttiiiner.l0hn H. Mack [22] Filed: Man 12, 1973 Assistant E.\'zlminer A. C. Prescott Attorney, Agent, or Ftrm-James J. Ralebate; James P. [21] Appl. 0 340,159 OSullivan; Jerome L. .leffers Related US. Application Data [621 Division of Ser. No. 819,526, April 24, 1969, 157] I ABSTRACT abandoned. Insulating liquids contaminated with suspended particles and dissolved material are regenerated by initially 1 1 Cl 0 204/180 removing the suspended particles by triboelectrie elec- 0/ 5; 210/500 tric filtration and thereafter removing the dissolved l l 1 (31-2 B011) F03C material by adsorption of the dissolved material onto a 0f Search R, suitablc Surfacc 7 Claims, No Drawings CLEANING PROCESS This is a division of application Ser. No. 819,526, filed Apr. 24, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a purification system, and more particularly, to the removal of particles and dissolved material in a liquid.

Numerous systems have been developed to separate suspended particles from a suspension. Suspensions, as employed herein, are defined as a system having particles suspended in a liquid. One type of suspension, called a colloidal suspension, contains particles too small for resolution with an ordinary light microscope. Although the invention hereinafter described will operate to separate particles from all types of suspensions, emphasis is placed on colloidal suspensions since they generally present a more difficult separation problem. Solutions, as employed herein, are defined as a homogeneous mixture of two or more materials subdivided down to a molecular scale, at least one of the materials being a liquid.

While this invention is applicable to a wide range of industrial fields, the following description of utility in the art of electrostatography is presented for illustrative purposes.

The process for the formation and development of images on the surface of photoconductive materials by electrostatic means is well known. These processes include dry techniques such as cascade, powder cloud, and magnetic brush processes and wet techniques such as the liquid development process. One conventional liquid development process involves placing a uniform electrostatic charge on a photoconductive insulating layer comprising zinc oxide powder and a resinous binder carried on a conductive paper substrate, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light to form an electrostatic latent image and developing the electrostatic latent image by depositing on the image a charged toner which is dispersed in an insulating liquid. Frequently, a resin or varnish is incorporated in the liquid developer to adjust the electric charge of the toner. Further, a dye or pigment is employed in the developer to provide the desired color.

Although considered a highly desirable technique for the formation of images, difficulties are encountered with attempts to form high quality images with the liquid development process. Generally, liquid development is effected by either immersing the electrostatic latent image bearing surface into the liquid developer or contacting the image bearing surface with a uniform film of liquid developer on the applicator surface. The liquid developer adhering to the imaging surface is thereafter dried. Unfortunately, the liquid developer adhering to the background areas of the imaging surface contains toner particles which remain on the imaging surface after drying. These undesirable background deposits are particularly acute in high speed continuous tone development systems employing a liquid developer containing high concentrations of toner particles and dissolved additives.

It has been found that uniform photoreceptor properties can be maintained and background toner deposits can be reduced by rinsing the imaging surface immediately after development with a liquid which contains little or no toner particles or other contaminants. However, the rinsing liquid rapidly becomes ineffective due to the accumulation of toner particles and other undesirable materials found in liquid developers. Generally, the rinsing or cleaning liquid is sufficiently insulating to prevent destruction of the deposited toner image. Further, the cleaning agent should be selected to be miscible with the liquid carrier employed in the liquid developer, should not unduly cause deterioration of the photoreceptor surface and should be capable of drying quickly. Generally, liquids that satisfy these requirements are relatively expensive. Further, they tend to dry quickly and the resulting vapors may escape from the development equipment thereby causing undesirable loss of the liquid as well as contamination of the ambient atmosphere. Typical cleaning liquids include isoparaffinic hydrocarbon solvents, tetrachlorodifluoroethane, kerosene, iso-octane and mixtures thereof. Since cleaning agents are expensive, they are recycled and used repeatedly to clean developed photoreceptor surfaces. Unfortunately, the cleaning liquid eventually becomes contaminated with materials from the developer such as toner particles, dyes, fixing agents, stabilizers, electric charge regulators and the like. These contaminants tend to cause deterioration of photoreceptor electrical properties thereby inhibiting the proper formation of subseqently developed images. In addition, the concentration of the toner particles in the cleaning liquid may reach the level where background toner deposits may be increased rather than decreased during the cleaning operation. Thus, it becomes necessary to remove the suspended toner and dissolved components present in the cleaning liquid to render it suitable for reuse.

Electrophoretic filtering is one effective method for filtering suspended particles from a cleaning liquid. As described above, the cleaning liquid is an insulating liquid and the toner particles colloidally suspended therein have an electric charge. If the cleaning liquid is interposed between two electrodes having a sufficiently high electric potential applied thereto, the toner particles are attracted to and deposit on either of the electrodes, depending on the polarity of charge thereon. Although this method is capable of removing colloidal particles having an electric charge, the resinous components, varnishes, dyes or the like which are dissolved in the cleaning liquid cannot be removed.

Another method for the filtration of cleaning liquid comprises passing a contaminated cleaning liquid through a filtering layer of triboelectrically chargeable solid particles which acquire an electric charge in the liquid cleaning agent. Thus, by a proper selection of the triboelectrically chargeable materials employed in the layer so that the material will acquire an electric charge of a polarity opposite to that of the electric charge of the toner suspended in the cleaning liquid, the toner particles may be removed from the cleaning liquid by deposition of the toner particles onto the surface of the triboelectrically charged particles. The selection of triboelectrically chargeable materials is well known in the electrostatic imaging art and the patent literature is replete with examples of materials which will acquire opposite electrical charges triboelectrically. Since removal of the toner particles from the cleaning liquid is effected by electrostatic means, the solid particles in the filtering layer as well as the voids occurring between the solid particles may be far greater in size than the toner particles. For example, the particles may have an average particle diameter in a range from about A; millimeter to about 3 millimeters. Although filtration may be effected with particle diameters outside this range, the smaller diameter particles cause a reduction in filtration speed and the larger diameter particles tend to remove fewer triboelectrically chargeable toner particles from the cleaning liquid. The particles in the filtering layer may have any suitable shape. Typical shapes include spheres, cylinders, platelets and gran ules. This filtration method is particularly desirable because it eliminates the need for an external electrical power source, as well as avoiding the need for high electrical potentials and the necessity for suction equipment. Unfortunately, this filtration system is incapable of effectively filtering resinous materials, varnishes, dyes and the like dissolved in a liquid cleaner. The dissolved contaminants in a cleaning liquid are particularly undesirable where a multi-colored image is to be formed on a photoconductive imaging surface. Generally, multi-colored images are formed by sequentially depositing a cyan colored toner, a yellow colored toner and a magenta colored tone. Each of these colorants are deposited by a series of operations including an electric charging step, an exposure step, a developing step and a cleaning step. The gradual formation of a thin film of dissolved contaminants on the photoreceptor surface during each cleaning step alters the electrical properties of the photoreceptor surface and causes degradation of image quality, particularly where the contaminant film is not deposited uniformly.

Attempts to simultaneously remove colloidally dispersed toner particles as well as dissolved material from a cleaning liquid by adsorption onto the surface of solid materials having very fine pores have not been especially successful. Apparently, the colloidal toner particles acquire an electric charge when suspended in the cleaning liquid and tend to adhere to the surface of materials employed to remove the suspended and dissolved contaminants from the liquid cleaning agent. Consequently, the very fine pores of the material employed to remove contaminants rapidly become clogged with the colloidal toner particles thereby markedly degrading the capacity of the material from removing dissolved contaminants by adsorption. Since most liquid purification techniques are deficient in one or more of the above areas, there is a continuing need for an improved liquid purification process.

SUMMARY OF THE INVENTION It is therefore, an object of this invention to provide a liquid purification process overcoming the above noted deficiencies.

It is another object of this invention to provide a liquid purification technique which purifies liquids at very high rates.

It is a further object of this invention to provide a liquid purification technique which more effectively removes suspended and dissolved contaminants.

It is still another object of this invention to provide a liquid purification technique which requires less power to effectuate purification.

It is another object of this invention to provide a liquid purification system superior to those of known systerns.

The above objects and others are accomplished by initially removing suspended contaminant particles from the cleaning liquid and thereafter removing dissolved contaminants by adsorption onto a suitable surface. Although many methods for removing suspended particles from a liquid are known, such as filtering through filter paper, optimum results are obtained with electrophoretic filtering techniques or triboelectric filtering techniques because of the high rate of filtration achieved and the low quantity of power consumed.

As discussed above, toner particles acquire an electrostatic charge when dispersed in an insulating cleaning liquid. Electrophoretic filtering utilizes this electrostatic charge on the toner particles to cause deposition of the toner particles from the suspension onto an electrode surface when the contaminated cleaning liquid is interposed between two electrodes having an electric field applied thereto. The electrophoretic filtering system is highly efficient and consumes a relatively small amount of electric current.

The triboelectric filtering technique described above is also very efficient and does not require an external source of electrical power. Any suitable solid particulate material which is substantially insoluble in the cleaning liquid may be employed in the triboelectric filtering layer. lt is well known that when solid particles are present in a given substantially non-conductive or insulating liquid, the solid particles are charged to either a positive or negative polarity. For example, when such solid matter as polyvinylchloride, polyethylene, polyvinylidene chloride, fluorinated resins or nitrocellulose is immersed in a mineral oil, the surface of such solid matter is charged negatively. Solid materials such as protein, polyamide resins, ethylcellulose or glass is charged positively in mineral oil. Thus, if mineral oil containing suspended particles having a positive charge are poured through a layer of polyvinylchloride pellets, the suspended particles will be attracted and deposit on the surface of the polyvinylchloride pellets. The mechanism of this phenomenon is such that Coulombs attraction generated by between the negative charge on the surfaces of the polyvinylchloride pellets and the positive charge of the suspended particles causes the latter to be attracted to and deposit on the surface of the polyvinylchloride pellets. Optimum results are obtained when the polyvinylchloride polymer comprises a rigid, straight chain polymer free of plasticizers which are soluble in the cleaning liquid because the plasticizers would contaminate the cleaning liquid. If the suspended toner particles in the cleaning liquid are negatively charged, the filtering particles may comprise, for example, gelatin flakes or nylon pellets. Obviously, other suitable materials may be selected from the well known triboelectric series to provide filtering particles which will acquire a charge having an opposite polarity from that of the toner particles upon immersion in the cleaning liquid. The effectiveness of the filtering particles is gradually reduced as the particles become covered with the electrically charged toner particles. Rapid regeneration of the filtering particles may be effected by mere mechanical washing or by washing with a solvent which dissolves the toner particles. If the filtering particles comprise heat resistant materials such as glass beads, the deposited toner particles may be decomposed by heating.

Unlike the principle of conventional filtration by filter paper which serves to mechanically impede the passage of suspended particles, triboelectric filtration relies upon the electrostatic attraction between the suspended particles and the surfaces of the filtering material. The filtering material in triboelectric filtration should have a greater particle size than the suspended particles to permit rapid passage of the contaminated liquid through the mass of filtering material particles. For example, the filtering material may have a particle size of about /a millimeter to about 3 millimeters while the suspended particles may have a colloidal size of less than about 1 micron. These sizes are such that if the suspended particles are not electrostatically attracted to the filtering material, it would normally pass through the spaces between the filtering material.

After the rapid removal of suspended particles from the cleaning liquid, removal of dissolved contaminants is effected by adsorption. Materials exhibiting adsorptive properties are well known in the art and are referred to as adsorbents". The surfaces of adsorbents contain very fine pores. Adsorption is believed to occur through the operation of van der Waals forces between the solid adsorbent and the dissolved molecules which are referred to in the art as adsorbates. The effective removal of an adsorbate from solution by an adsorbent depends on a number of factors including contact time, the available adsorbent surface area, the specific combination of adsorbate and adsorbent materials employed and the temperature at which the separation is conducted. The prior art is replete with information regarding the foregoing factors. Typical materials employed to remove dissolved material by adsorption include fine particles of clay, alumina, silica, magnesia,

titanium oxide, zinc oxide, molecular sieves and the like. Molecular sieves comprise a special class of adsorbents. These materials contain atoms arranged in a crystal lattice containing a large number of small cavities interconnected by smaller openings or pores of precisely uniform size. When dried, these materials will adsorb other molecules that are small enough to pass through the pores. Typical molecular sieve materials include zeolites, cross-linked dextran and other similar materials. Generally, zeolites comprise hydrated silicates of aluminum and, in addition, sodium and/or calcium.

Thus, the removal of suspended particles by filtration, particularly with the aid of electric fields, followed by the removal of dissolved contaminants with the aid of. an adsorbent, permits more rapid regeneration of a cleaning liquid contaminated by conventional liquid developer suspended and dissolved components than when either the filtration regenerating or the adsorption regenerating techniques are employed alone. Further, the improved regenerating technique of this invention extends the total life of the filtering materials.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further define, describe and compare preferred methods and materials of the present invention. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I An electrostatic latent image formed on an imaging surface of a zinc oxide binder plate is briefly immersed in kerosene and thereafter developed with a liquid deblack offset ink. The commercial black offset ink comprises a dye, rosin-modified phenolformaldehyde resin varnish and a pigment. Upon mixing of the foregoing ingredients, the resulting developer contains dissolved rosin-modified phenol-formaldehyde resin varnish as well as colloidal particles comprising the pigment particles, dye and rosin-modified phenol-formaldehyde resin varnish adsorbed on the surface of the pigment particles.

Development of the electrostatic latent image is effected by immersion of the binder plate in the liquid developer for about 1 minute. Upon removal of the binder plate, it is found that the developed plate is coated on the imaging surface as well as the rear surface with a uniform, thin layer of liquid developer. The deposited layer of liquid developer is then dried on the surface of the plate. A heavy deposit of toner particles is observed on the background areas of the surface as well as on the rear surface of the plate.

EXAMPLE II The process described in control Example I is repeated except that prior to drying the developed surface of the binder plate, the plate is gently washed by immersing it in a bath of about 1 liter of isoparaffin available from the Esso Company. During the washing step, the liquid developer adhering to the background areas of the binder plate diffuses into the cleaning agent. The plate is thereafter removed from the cleaning bath and dried by exposure to a current of warm air. Upon repeating the development and washing steps with 10 additional binder plates, the cleaning liquid changes from a clear transparent liquid to a conspicuously colored liquid having markedly reduced cleaning power. Since the rosin-modified phenolformaldehyde resin varnish is more soluble in the isoparaffin cleaning liquid than in the cyclohexane carrier liquid, rapid contamination of the cleaning liquid with the varnish is observed. The contaminated cleaning liquid is then subjected to an electrophoretic regeneration treatment. In conducting the electrophoretic regeneration treatment, an electrophoretic device is employed comprising a stack of 101 metallic plates having a thickness of about 1 millimeter and sides of about 20 centimeters by about 30 centimeters, each metallic plate being separated from each adjacent metallic plate by a distance of about 2 millimeters. Every other plate inthe stack is connected electrically to one electrode of a source of DC potential and the remaining plates are connected to the other electrode of the DC potential source. The stack is inserted in a container having an inside dimension of about 20 centimeters by about 30 centimeters by about 30 centimeters. After the container is filled with the contaminated cleaning liquid, a DC potential of about 1,000 volts is applied to the alternately connected metal plates. Complete removal of the toner particles from the cleaning liquid is effected in from about 3 to about 10 seconds. Because the toner particles are positively charged in the above described developer, these particles are attracted and deposited on the plate connected to the cathode of the DC potential source. The regenerated liquid is drained through a valve at the bottom of the container while the electrical potential remains applied to the plates. The resulting regenerated liquid is transparent and free of suspended toner particles. Unfortunately, the regenerated liquid is characterized by yellow tint due to the presence of dissolved varnish. This partially regenerated cleaning liquid is then employed to wash freshly developed imaging surfaces. Although the resulting washed imaging surfaces have reduced background toner deposits, the deposited varnish coating on the imaging surface interferes with subsequently deposited toner particles when the imaging surface is imaged two additional times in a multi-color process. Particularly poor image quality is obtained when the deposited varnish coating is nonuniform.

EXAMPLE Ill The process described in control Example ll is repeated except that the partially regenerated cleaning liquid is subjected to a second regeneration treatment which removes the dissolved varnish. The second regeneration treatment is effected with the aid of a device comprising a metallic cylinder having an inside diameter of about 20 centimeters and a height of about 40 centimeters. A valve is provided at the bottom of the cylinder to permit removal of treated liquid. The cylinder is filled to about 80 percent of its capacity with about 30 to 60 mesh particles of crystalline metal alumina silicate (molecular sieve material 13 X available from the Linde Company.) These particles contain tiny pores having a nominal pore diameter of about angstroms. The regenerated carrier liquid containing the dissolved varnish is poured into the top of the cylinder and removed through the valve at the bottom of the cylinder. The liquid removed from the bottom of the cylinder is substantially free of any dissolved varnish. Some entrained molecular sieve particles, however, are observed in the regenerated liquid initially removed from the cylinder. These entrained particles may easily be electrophoretically removed from the cleaning liquid by the technique described in EXAMPLE II.

EXAMPLE IV A cleaning liquid contaminated with toner and varnish in substantially the same manner as described in Example II is treated in a triboelectric filtering device. This device comprises a metallic cylinder having an inside diameter of about centimeters and a height of about 100 centimeters. A ZOO-mesh metallic screen is positioned at approximately the middle of the cylinder and a similar metallic screen is positioned at the bottom of the cylinder. An outlet is provided at the bottom of the cylinder below the lower metallic screen. The lower half of the cylinder is filled with the same molecular sieve material described in Example III while the upper half of the cylinder is filled with brass shavings. These brass shavings range in size from about A; millimeter to about 3 centimeters in length and from about microns to about millimeter in thickness. When the contaminated cleaning liquid is poured into the top of the cylinder, the brass shavings become negatively charged upon contact with the cleaning liquid. Thus, the positively charged toner particles are attracted to and deposit on the surface of the brass filings during passage of the liquid through the upper half of the cylinder. The dissolved varnish is removed by adsorption during passage of the cleaning liquid through the lower half of the cylinder. The regenerated liquid removed from the outlet at the bottom of the cylinder is colorless and transparent.

EXAMPLE V A metallic cylinder having an inside diameter of about 20 centimeters and a height of about 120 centimeters is divided into three equal portions along its height by 200-mesh metallic screens positioned inside the cylinder. The uppermost portion is filled with polyvinyl chloride shavings, the middle portion with a layer of about 30 to about mesh particles of molecular sieve material 13 X available from Linde Company, and the lowermost portion with a layer of a mixture of polyvinyl chloride particles and glass beads having a diameter of from about 0.3 millimeters to about 0.4 millimeters. An outlet is provided at the bottom of the cylinder. The polyvinyl chloride particles are granular and have an average diameter ranging from about A; millimeter to about 3 millimeters. A contaminated cleaning liquid formed by a process substantially identical to the process described in Example II is introduced at the top of the cylinder. The suspended toner particles are triboelectrically removed from the contaminated cleaning liquid by the polyvinyl chloride particles. The dissolved varnish is removed from the cleaning liquid in the middle portion of the cylinder by adsorption onto the molecular sieve material. As the treated clear and colorless cleaning liquid continues into the lowermost portion of the cylinder with entrained molecular sieve particles, the cleaning liquid causes the glass beads to be positively charged and the polyvinyl chloride particles to be negatively charged. Thus, any colloidal molecular sieve particles originating from the middle portion of the cylinder will be removed by either the charged glass beads or the charged polyvinyl chloride particles depending upon the polarity of charge carried by the molecular sieve particles.

EXAMPLE VI The procedure described in Example V is repeated except that the adsorbent is replaced with particles of copper sulfate free from water of crystallization. The resulting regenerated liquid is also clear and colorless.

Instead of contacting the adsorbent with a cleaning liquid containing dissolved adsorbate material by passing the contaminated cleaning liquid through a layer of adsorbent particles, one may thoroughly mix finely divided adsorbent material with the contaminated cleaning liquid, thereafter allow the adsorbent to settle upon standing and finally filtering the supernatant fluid. Any colloidally suspended adsorbent particles may be removed with the aid of an electric field as described above. Typical adsorbent materials include finely divided clay, zinc oxide, titanium oxide, alumina, colloidal silica and the like. Although this technique is less effective than passing contaminated liquid through a layer of adsorbent particles to remove dissolved contaminants such as varnish, other resins or dyes, the degree of adsorbate removal is sufficient to provide regenerated cleaning liquids which perform the cleaning operation satisfactorily.

Although specific materials and conditions are set forth in the foregoing examples, these are merely intended as illustrations of the present invention. Various other suitable triboelectric filtering materials, adsorption filtering materials, cleaning liquids, and developer materials such as those listed above may be substituted for those in the examples with similar results. Other materials may also be added to the regeneration materials, cleaning liquid or developer to sensitize, synergise or otherwise improve the efficiency or other desirable properties of the system.

Other modifications of the. present invention will ocur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

l. A method for separating suspended solid particles and dissolved absorbate material from an insulating liquid wherein the liquid is contaminated with toner particles, dyes, fixing agents, stabilizers or electric charge regulators deposited in the liquid by contacting it with a xerographic imaging surface after said surface has been developed by contacting it with a liquid developer comprising removing said suspended particles from said insulating liquid by contacting said insulating liquid and said suspended solid particles with a particulate filtering material of particle size less than 1 mm capable of acquiring an electrical charge upon contact with said insulating liquid, said electrical charge having a polarity opposite the polarity of the electrical charge acquired by said suspended particles in said insulating liquid whereby said suspended particles are attracted to and deposit on said filtering material.

2. A method according to claim 1 including removing said suspended particles from said insulating liquid by triboelectric filtration.

3. A method according to claim 1 wherein said filtering material comprises a layer of filtering material particles having a particle size larger than said suspended particles and containing voids sufficiently large to permit free passage of said suspended particles through said layer.

4. The method of claim 1 wherein the suspended particles have a colloidal size of less than about 1 micron.

5. The method of claim 1 wherein the suspended solid particles are in the shape of spheres, cylinders, plateletts or granules.

6. The method of claim 1 wherein the suspended solid particles are made up of polyvinylchloride, polyethylene, polyvinylidene chloride, fluorinated resins or nitrocellulose and the insulating liquid is a mineral oil.

7. The method of claim 1 wherein the suspended solid particles are made up of protein, a polyamide resin, ethyleellulose or glass. 

1. A METHOD FOR SEPARATING SUSPENDED SOLID PARTICLES AND DISSOLVED ABSORBATE MATERIAL FROM AN INSULATING LIQUID WHEREIN THE LIQUID IS CONTAMINATED WITH TONER PARTICLES, DYES, FIXING AGENTS, STABILIZERS OR ELECTRIC CHARGE REGULATORS DEPOSITED IN THE LIQUID BY CONTACTING IT WITH A XEROGRAPHIC IMAGING SURFACE AFTER SAID SURFACE HAS BEEN DEVELOPED BY CONTACTING IT WITH A LIQUID DEVELOPER COMPRISING REMOVING SAID SUSPENDED PARTICLES FROM SAID INSULATING LIQUID BY CONTACTING SAID INSULATING LIQUID AND SAID SUSPENDED SOLID PARTICLES WITH A PARTICULATE FILTERING MATERIAL OF PARTICLE SIZE LESS THAN 1 MM CAPABLE OF ACQUIRING AN ELECTRICAL CHARGE UPON CONTACT WITH SAID INSULATING LIQUID, SAID ELECTRICAL CHARGE HAVING A POLARITY OPPOSITE THE POLARITY OF THE ELECTRICAL CHARGE ACQUIRED BY SAID SUSPENDED PARTICLES IN SAID INSULATING LIQUID WHEREBY SAID SUSPENDEDED PARTICLES ARE ATTRACTED TO AND DEPOSITON SAID FILTERING MATERIAL.
 2. A method according to claim 1 including removing said suspended particles from said insulating liquid by triboelectric filtration.
 3. A method according to claim 1 wherein said filtering material comprises a layer of filtering material particles having a particle size larger than said suspended particles and containing voids sufficiently large to permit free passage of said suspended particles through said layer.
 4. The method of claim 1 wherein the suspended particles have a colloidal size of less than about 1 micron.
 5. The method of claim 1 wherein the suspended solid particles are in the shape of spheres, cylinders, plateletts or granules.
 6. The method of claim 1 wherein the suspended solid particles are made up of polyvinylchloride, polyethylene, polyvinylidene chloride, fluorinated resins or nitrocellulose and the insulating liquid is a mineral oil.
 7. The method of claim 1 wherein the suspended solid particles are made up of protein, a polyamide resin, ethylcellulose or glass. 